1. Field of the Invention
The present invention relates to a composition for film formation, which can be formed into a porous film that excels in-dielectric properties, adhesion, film consistency and mechanical strength, and has reduced absorption; a porous film and a method for forming the same; and a semiconductor device, which contains the porous film inside.
2. Description of the Related Art
In the fabrication of semiconductor integrated circuits, as the circuits are packed tighter, an increase in interconnection capacitance, which is a parasitic capacitance between metal interconnections, leads to an increase in interconnection delay time, thereby hindering the enhancement of the performance of semiconductor circuits. The interconnection delay time is called an RC delay, which is in proportion to the product of the electric resistance of the metal interconnections and the capacitance between the interconnections. Reducing the interconnection delay time requires reducing the resistance of metal interconnections or the interconnection capacitance. The reduction in the interconnection capacitance can prevent a densely packed semiconductor device from causing an interconnection delay, thereby realizing faster semiconductor device with reduced power consumption.
One method for reducing interconnection capacitance is to reduce the relative permittivity (dielectric constant) of an interlevel insulator film disposed between metal interconnections. As such an insulator film with a low relative permittivity, it has been considered to use a porous film instead of a silicon oxide film, which has been used conventionally. A porous film can be said to be the only practical film as a material which has potential to have a relative permittivity (dielectric constant) of 2.0 or less, and various methods for forming a porous film have been proposed.
In a first method for forming a porous film as described in Japanese Patent Provisional Publication Nos. 2001-2993, 2001-98218 and 2001-115021, a composition comprising a thermally unstable organic resin component and a siloxane polymer is synthesized, is applied on the substrate to form a coating film, and then treated thermally for decomposing and volatilizing the organic resin component so that a number of pores are formed in the film.
In a second method for forming a porous film as described in Japanese Patent Provisional Publication Nos. 2001-131479 and 2001-80915, it is known to carry out processing as follows: a silica sol solution is applied onto a substrate by coating or using a CVD method so as to form a wet gel; and then the silica sol is subjected to a condensation reaction while restricting volume reduction by controlling the speed of the evaporation of the solvent from the wet gel.
In a third method for forming a porous film as described in WO 00/12640 pamphlet, it is known that a silica micro-particle solution is applied on a substrate to form a coating film, and then the coating film is sintered to form a number of micro-pores between silica micro-particles.
However, these methods have respective major drawbacks as follows.
In the first method for forming a porous film, although the high compatibility between the siloxane polymer and the organic component is required from the coating step to the hardening step in order to make the pores small, there is a problem that the compatibility between the siloxane polymer and the organic resin is poor.
In the second method for forming a porous film, the speed control of the evaporation of the solvent from the wet gel requires a special type of coating device, which increases the cost. In addition, a significant amount of silanol remains on the surface of the micro-pores, which must be silanized because otherwise hygroscopicity is high and the film quality decreases. The silanization makes the process more complicated. In the case where a wet gel is formed by the CVD process, it is necessary to use a special type of CVD device, which is different from the plasma CVD device generally used in the semiconductor process, thereby also increasing the cost.
In the third method for forming a porous film, because the diameter of the pores is determined by the accumulation structure of the silica micro-particles, it becomes very large. This makes it difficult to set the relative permittivity (dielectric constant) of the porous film to 2 or below.
As mentioned above, because the conventional material makes the pore diameter larger during the formation of porous film, there is a problem that the film has difficult in having low dielectric constant. There is also a problem that the cost for forming porous film having fine pores becomes higher. Further, when porous film produced by conventional material is used as an insulator film in multi-level interconnects of the semiconductor device, there is a problem that the mechanical strength required for the semiconductor processing is not obtained.
Thus, when the dielectric constant of the porous film used as an insulator film in multi-level interconnects of the semiconductor device is high, the RC delay increases. Consequently, there is a large problem that the performance of the semiconductor device (high speed and low power consumption) has not been improved. There is also a problem that the low mechanical strength of the porous film leads to the lowered reliability of the semiconductor device.